Apparently the reporter was given access to McChrystal for 30 days. IIRC that is unprecedented. Over that period of time it would be very easy to momentarily let your guard down a few times.
I have trouble believing that McChrystal would voluntarily allow a reporter 30 days access. It had to be on orders from Obama. Also, McChrystal may have decided that since they were forcing him to give that much access, he would REALLY give them access and let the cards fall where they may.
According to a post at BLACKFIVE, the reporter was originally only supposed to be there for 2 days but the Icelandic volcano eruption extended his visit for a few more weeks.